That might seem like a non sequitur, but considerableresearch has linked green space with better human health. A recent study provides new evidence for this beneficial connection, along with at least a partial explanation for its power.

The study finds that people who live in leafier areas have lower levels of several stress-related biomarkers, including adrenaline. In addition, they have an enhanced ability to grow and repair blood vessels.

"Increasing the amount of vegetation in a neighborhood may be an unrecognized environmental influence on cardiovascular health," said lead author Aruni Bhatnagar, professor of medicine at the University of Louisville. He called such an effort "a potentially significant public-health intervention."

The study, in the Journal of the American Heart Association, featured 408 people recruited at a preventive cardiology clinic in Louisville, Kentucky. All were either diagnosed with, or considered at high risk of, cardiovascular disease.

The researchers measured the impact of stress on their bodies using a variety of measures, including the level of the hormone epinephrine (a.k.a. adrenaline), which is released as part of the well-known fight-or-flight response. Additionally, in a subgroup of 255 participants, they measured the level of specific blood cells that are key to building and repairing vessels.

Using satellite imagery, they recorded the amount of green space in the immediate vicinity of their homes (up to 0.6 miles from their door).

"Residential proximity to vegetation was associated with cardiovascular health, as reflected by a range of biomarkers of cardiovascular injury and disease risk," the researchers report.

Specifically, people living near green spaces had lower levels of stress-activated hormones including adrenaline, and higher levels of those specialized blood cells that help keep the vascular system healthy.

"Individuals living in greener areas are likely to have better wound-healing response, and higher capacity to repair blood vessels," they write.

Some of these findings, including the reduction in adrenaline levels, were more pronounced in women. Importantly, the overall pattern held true regardless of age, race, the socioeconomic status of their neighborhood, and whether or not the subjects smoked cigarettes or used statins. So it's not just a matter of wealthier people having better access to greener neighborhoods. Vegetation appears to be an equal-opportunity health promoter.

Precisely why stepping out your door and into nature lowers stress isn't fully understood. Researchers have speculated that it encourages more outdoor exercise, as well as more emotion-soothing socializing with neighbors.

A 2017 study from Germany focused on the amygdala, a part of the brain that plays a key role in processing emotions, including anxiety. It found this region was healthier in people who lived near forests. A better ability to deal with upsetting news means putting less stress into your body, where it can ultimately damage your heart.

Given the evidence that green space also helps children's cognitive development, ensuring easy access to nature would seem to be an inexpensive, highly beneficial public policy goal. To update the mantra of Kermit the Frog, it's not easy being without green. For some, it can even be heartbreaking.